Abstract : The purpose of this study was to add a secondary coolant loop model to the Space Nuclear Power Systems Analysis Model (SNPSAM). The heat rejection systems, including the TEM pump, energy conversion assembly, and radiator, are emphasized while the reactor model is de-emphasized. Modifications were made to the TEM pump and heat exchanger models. A subroutine that simulated the secondary was written and SNPSAM was modified to incorporate this subroutine. Modifications included subroutines for computing head loss, heat transfer coefficients, and mass transfer. In the revised SNPSAM, the steady-state mode generated rejected power outputs that defied an energy balance in the system. The cause for the imbalance is in the iterative method used by the thermocouple model. The iterative method used in the thermocouple model made SNPSAM sensitive to initial conditions. In the transient mode operation, the model generated solutions that were accurate but displayed oscillations in the first few seconds of simulation. The steady-state performance predicted by SNPSAM was compared with the predicted steady-state performance. Since the steady-state mode would not function satisfactorily, a slowly varying transient case was used in its place. Emphasis is given to procedures that facilitate tailoring the model to whatever design is current.